Support processing of JOSE extensions.

check.go

@@ -17,6 +17,20 @@ var ErrSigMiss = errors.New("jwt: signature mismatch")
 
 var errPart = errors.New("jwt: missing base64 part")
 
+// “Producers MUST NOT use the empty list "[]" as the "crit" value.”
+// — “JSON Web Signature (JWS)” RFC 7515, subsection 4.1.11
+var errCritEmpty = errors.New("jwt: empty array in crit header")
+
+// EvalCrit is invoked by the Check functions for each token with one or more
+// JOSE extensions. The crit slice has the JSON field names (for header) which
+// “MUST be understood and processed” according to RFC 7515, subsection 4.1.11.
+// “If any of the listed extension Header Parameters are not understood and
+// supported by the recipient, then the JWS is invalid.”
+// The respective Check function returns any error from EvalCrit as is.
+var EvalCrit = func(token []byte, crit []string, header json.RawMessage) error {
+	return fmt.Errorf("jwt: unsupported critical extension in JOSE header: %q", crit)
+}
+
 // ParseWithoutCheck skips the signature validation.
 func ParseWithoutCheck(token []byte) (*Claims, error) {
 	var c Claims
@@ -146,9 +160,9 @@ func (c *Claims) scan(token []byte) (firstDot, lastDot int, sig []byte, alg stri
 	}
 
 	var header struct {
-		Kid  string        `json:"kid"`
-		Alg  string        `json:"alg"`
-		Crit []interface{} `json:"crit"`
+		Kid  string   `json:"kid"`
+		Alg  string   `json:"alg"`
+		Crit []string `json:"crit"`
 	}
 	if err := json.Unmarshal(buf[:n], &header); err != nil {
 		return 0, 0, nil, "", fmt.Errorf("jwt: malformed JOSE header: %w", err)
@@ -158,12 +172,13 @@ func (c *Claims) scan(token []byte) (firstDot, lastDot int, sig []byte, alg stri
 
 	alg = header.Alg
 	c.KeyID = header.Kid
-	// “If any of the listed extension Header Parameters are not understood
-	// and supported by the recipient, then the JWS is invalid. […]
-	// Producers MUST NOT use the empty list "[]" as the "crit" value.”
-	// — “JSON Web Signature (JWS)” RFC 7515, subsection 4.1.11
 	if header.Crit != nil {
-		return 0, 0, nil, "", fmt.Errorf("jwt: unsupported critical extension in JOSE header: %q", header.Crit)
+		if len(header.Crit) == 0 {
+			return 0, 0, nil, "", errCritEmpty
+		}
+		if err := EvalCrit(token, header.Crit, c.RawHeader); err != nil {
+			return 0, 0, nil, "", err
+		}
 	}
 
 	// signature

check_test.go

@@ -6,6 +6,8 @@ import (
 	"crypto/ed25519"
 	"crypto/rand"
 	"crypto/rsa"
+	"encoding/json"
+	"errors"
 	"strings"
 	"testing"
 )
@@ -208,13 +210,50 @@ func TestCheckHashNotLinked(t *testing.T) {
 
 func TestJOSEExtension(t *testing.T) {
 	// “Negative Test Case for "crit" Header Parameter” from RFC 7515, appendix E.
-	const token = "eyJhbGciOiJub25lIiwNCiAiY3JpdCI6WyJodHRwOi8vZXhhbXBsZS5jb20vVU5ERUZJTkVEIl0sDQogImh0dHA6Ly9leGFtcGxlLmNvbS9VTkRFRklORUQiOnRydWUNCn0.RkFJTA."
+	const testToken = "eyJhbGciOiJub25lIiwNCiAiY3JpdCI6WyJodHRwOi8vZXhhbXBsZS5jb20vVU5ERUZJTkVEIl0sDQogImh0dHA6Ly9leGFtcGxlLmNvbS9VTkRFRklORUQiOnRydWUNCn0.RkFJTA."
+	errTest := errors.New("test error")
 
-	_, err := ParseWithoutCheck([]byte(token))
+	_, err := ParseWithoutCheck([]byte(testToken))
 	const want = "jwt: unsupported critical extension in JOSE header: [\"http://example.com/UNDEFINED\"]"
 	if err == nil || err.Error() != want {
 		t.Errorf("got error %q, want %q", err, want)
 	}
+
+	bu := EvalCrit
+	defer func() {
+		EvalCrit = bu // restore
+	}()
+	// extend
+	EvalCrit = func(token []byte, crit []string, header json.RawMessage) error {
+		if string(token) != testToken {
+			t.Errorf("got token %q, want %q", token, testToken)
+		}
+
+		const wantHeader = "{\"alg\":\"none\",\r\n \"crit\":[\"http://example.com/UNDEFINED\"],\r\n \"http://example.com/UNDEFINED\":true\r\n}"
+		if string(header) != wantHeader {
+			t.Errorf("got header %q, want %q", header, wantHeader)
+		}
+
+		const wantCrit = "http://example.com/UNDEFINED"
+		if len(crit) != 1 || crit[0] != wantCrit {
+			t.Errorf("got crit %q, want %q", crit, wantCrit)
+		}
+
+		return errTest
+	}
+	_, err = ParseWithoutCheck([]byte(testToken))
+	if err != errTest {
+		t.Errorf("got error %q, want %q", err, errTest)
+	}
+
+	token, err := new(Claims).HMACSign(HS256, []byte("secret"),
+		json.RawMessage(`{ "crit": [] }`))
+	if err != nil {
+		t.Fatal("compose token with empty crit array:", err)
+	}
+	if _, err := HMACCheck(token, []byte("wrong")); err != errCritEmpty {
+		t.Errorf("got error %q, want %q", err, errCritEmpty)
+	}
 }
 
 func TestErrPart(t *testing.T) {